Technology which satisfies the demand to grasp and manage in real time the positions and other information of plural mobile radio stations has conventionally been desired, without having to describe the example of radio-dispatched taxis, and is already being utilized as mobile object information sharing systems. In this case, usually both the base station and the mobile stations use one frequency that is the same in order to ensure that the number of radio frequencies used does not increase.
For example, in Patent Document 1, a data transmission system is disclosed where, in a conventional mobile object information sharing system in which mobile objects mutually use the same radio frequency, in order to avoid interference resulting from plural stations transmitting at the same frequency at the same time, a shared standard time base is disposed in plural mobile radio stations (also called terminal stations) and mutually different times are allocated to the terminal stations as unique transmission timings on the standard time base, whereby time division communication that avoids overlapping of the transmission timings is conducted without requiring control of the communication from the base station.
However, the position information of the mobile objects must of course be transmitted at more frequent intervals when the mobile objects are moving at a high speed than when the mobile objects are moving at a low speed or are stopped. Yet in the case of the above system, the timings when the stations can transmit their position information are determined beforehand. For this reason, there has been the problem that when a mobile object in motion wishes to provide the position information of its own station at more frequent intervals than the ordinary time intervals, the mobile object cannot change the transmission period or timing as needed, must wait for the communication of non-urgent stations and cannot transmit the position information until its turn, and cannot timely provide effective position information because its position ends up changing greatly during that time.
Patent Document 1: JP-A-08-130774 (pp. 3-5; FIG. 1)